a) Around $5,000 - if successful something like $2,500-$3,500 if not. Most do it based on a fixed price.
b) Not to my knowledge! If you hire “senior software dev” you must pay what they consider to be fair market compensation - including pretty high salaries. The good thing: no one cares UNTIL you made it through the lottery!! So: first application, then lottery, which happens only once a year around April 1st, then the real process (and salary, etc)… good luck!
If the candidate already has a visa, you should reach out to a lawyer. USCIS is following new regulation where each transfer application has to independently meet criterion for the new job. Previously they used to consider approved H1B as a strong evidence of meeting criterion